Field of the Invention
Embodiments of the invention relate to an active matrix organic light emitting display.
Discussion of the Related Art
An active matrix organic light emitting display includes organic light emitting diodes (OLEDs) that self emit light and has a fast response time, a high light emitting efficiency, a high luminance, a wide viewing angle, and the like. The OLED includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode and cathode electrodes.
Further, the organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, a light emitting layer EML, an electron transport layer ETL, and an electron injection layer EIL. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer HTL and electrons passing through the electron transport layer ETL move to the light emitting layer EML and form excitons. As a result, the light emitting layer EML generates visible light.
In addition, the organic light emitting display arranges subpixels each including the OLED in a matrix form and adjusts an amount of current flowing in the OLED, thereby representing a grayscale. As shown in FIGS. 1 to 4, the OLED includes a plurality of unit pixels UPXL so as to represent a desired color. Each unit pixel UPXL includes four subpixels each representing a different color, i.e., a first subpixel having a red (R) OLED, a second subpixel having a green (G) OLED, a third subpixel having a blue (B) OLED, and a fourth subpixel having a white (W) OLED.
The unit pixels UPXL refresh a display image in each frame and implement a desired image. In this instance, in each frame, the unit pixels UPXL go through an initialization process by an initialization voltage Vinit and go through a programming process for an image refresh when the initialization voltage Vinit is applied to them. For the initialization and programming processes, the vertically adjacent unit pixels UPXL are connected to the same initialization voltage supply channel and receive the initialization voltage Vinit from a data driving circuit.
For example, as shown in FIGS. 1 and 4, vertically adjacent unit pixels UPXL on one line (for example, a first line) may be connected to a first initialization voltage supply channel CH1, vertically adjacent unit pixels UPXL on other line (for example, a second line) may be connected to a second initialization voltage supply channel CH2, and vertically adjacent unit pixels UPXL on other line (for example, a third line) may be connected to a third initialization voltage supply channel CH3.
The initialization and programming processes of the unit pixels UPXL are performed by sensing signals SEN and scan signals SCAN shown in FIG. 2. The sensing signals SEN are sequentially supplied to horizontal pixel lines through a line sequential manner. The scan signals SCAN are similarly applied. For example, as shown in FIG. 2, a scan signal SCAN(n−1) and a sensing signal SEN(n−1) may be supplied to an (n−1)th horizontal pixel line L(n−1), and a scan signal SCAN(n) and a sensing signal SEN(n) may be supplied to an nth horizontal pixel line L(n). As shown in FIG. 3, the sensing signal SEN turns on second switch TFTs ST2 included in the unit pixels UPXL and thus causes the initialization voltage Vinit received from the initialization voltage supply channel CH2 to be applied to the R, W, G, and B subpixels of the corresponding unit pixel UPXL.
A so-called sensing signal overlap drive method successively shifts sensing signals SEN to overlap each other by a predetermined period of time so as to secure a sufficient initialization period. FIG. 2 shows an example of the sensing signal overlap drive method. More specifically, FIG. 2 shows that the sensing signal SEN(n−1) and the sensing signal SEN(n) overlap each other by one horizontal period 1H (=one frame period/vertical resolution).
FIG. 3 shows a charge path of the initialization voltage Vinit in the overlap period ‘1H’ shown in FIG. 2. As shown in FIG. 3, a second unit pixel UPXL2 of the (n−1)th horizontal pixel line L(n−1) and a first unit pixel UPXL1 of the nth horizontal pixel line Ln, which are vertically adjacent to each other, simultaneously receive the initialization voltage Vinit in the overlap period ‘1H’ shown in FIG. 2.
However, in the sensing signal overlap drive, when a short circuit defect is generated in one of the subpixels belonging to a first unit pixel UPXL1 disposed on a predetermined horizontal pixel line, not only the remaining subpixels of the first unit pixel UPXL1, which receive the initialization voltage Vinit at the same time as the defective subpixel, but also the subpixels belonging to the second unit pixel UPXL2 vertically adjacent to the first unit pixel UPXL1 are affected by the short circuit defect. This is because the first unit pixel UPXL1 and the second unit pixel UPXL2 simultaneously operate during a predetermined period of time due to the sensing signal overlap drive.
For example, as shown in FIG. 3, a short circuit black spot (for example, a defect appearing when a green OLED does not emit light because of the short-circuit between both terminals of the green OLED) may be generated in a green (G) subpixel of a first unit pixel UPXL1 connected to an initialization voltage supply channel CH2 among the unit pixels UPXL disposed on the nth horizontal pixel line Ln.
In this instance, a low potential cell driving voltage EVSS less than the initialization voltage Vinit is applied to a source electrode of a driving thin film transistor (TFT) DT of each of red (R), white (W), and blue (B) subpixels of the first unit pixel UPXL1 in an initialization period for the initialization process and a programming period for data entry (the overlap period between the scan signal SCAN(n) and the sensing signal SEN(n) in FIG. 2). An amount of light emitted by each subpixel depends on a voltage Vgs between a gate electrode and a source electrode of the driving TFT DT, which is set in the programming period.
As described above, when a potential of the source electrode of the driving TFT DT is less than the initialization voltage Vinit in the programming period, the gate-source voltage Vgs of the driving TFT DT, which is set in the programming period, is greater than a desired value. Hence, the R, W, and B subpixels of the first unit pixel UPXL1 represent a luminance greater than a desired luminance. This problem is equally generated in R, W, G, and B subpixels of a second unit pixel UPXL2, which is vertically adjacent to the first unit pixel UPXL1 of FIG. 3 and is connected to the initialization voltage supply channel CH2.
FIG. 4 shows that luminances of the first and second unit pixels UPXL1 and UPXL2 are greater than a luminance of other unit pixel UPXL3 except the black spot resulting from the short circuit defect of the first unit pixel UPXL1. Further, in the existing sensing signal overlap drive method, in which sensing signals each having a pulse width of N horizontal periods NH (where N is a positive integer equal to or greater than 2) are shifted in the line sequential manner while overlapping each other by (N−1) horizontal period (N−1)H, N unit pixels, which are driven to overlap each other in response to the sensing signal among vertically adjacent unit pixels, are commonly connected to the same initialization voltage supply channel.
Therefore, when the short circuit defect is generated in one of the N unit pixels, the remaining unit pixels on other horizontal line vertically adjacent to the defective unit pixel are affected by the short circuit defect and represent an undesired luminance.